In 2000, the National Heart, Lung, and Blood Institute (NHLBI) competitively awarded contracts to conduct a randomized, double-blind, placebo-controlled trial in young children with sickle cell disease (SCD) to test the hypothesis that Hydroxyurea (HU) can prevent the onset of chronic end organ damage in children recruited before two years of age. The study was also designed to monitor clinical responsiveness to study treatments, to assess growth and development, and to monitor for toxicity from study treatments. The trial enrolled 193 subjects with SCD between the ages of 9 and 18 months from October 2003 to June 2007. Subjects remained on study drug for a period of two years. The purpose of this renewal is to perform structured long-term clinical, radiographic, and laboratory follow-up of children enrolled in the original Pediatric Hydroxyurea Phase III Clinical Trial (BABY HUG) and the Follow-Up Study I through the first decade of life. In addition, the Follow-Up Study II will provide evaluation of hydroxyurea effects on organ function or toxicity during long term use. In collaboration with the NICHD under the Best Pharmaceuticals for Children Act (BPCA) NHLBI, as the IND sponsor, will submit clinical data to the Food and Drug Administration (FDA) to seek pediatric use labeling for HU.